1. Field of the Invention
The present invention relates to an ink jet pigment ink (that hereinafter may also be referred to as “pigment ink” merely), by which a resulting recorded article exhibits high color development even when high-speed printing is conducted by means of a high-density nozzle and high ejection stability is obtained, and to an ink set.
2. Related Background Art
An ink-jet recording method is a technique wherein minute droplets of a recording liquid (ink) are ejected to apply them to a recording medium such as paper, thereby conducting recording. There is a method such that electrothermal converters are used as ejection-energy supply means in particular to apply thermal energy to an ink so as to generate bubbles, thereby ejecting droplets of the ink (see Japanese Patent Publication No. S61-59911). According to this method, the utilization of a high-density multi-orifice in an ink-jet recording head can be realized with ease, and high-resolution and high-quality images can be recorded at high speed.
On the other hand, a coloring material contained in an ink has heretofore been mainly on a dye dissolvable in an ink solvent, but an investigation with a view toward using a pigment from the viewpoints of light fastness and water fastness has been advanced. However, the pigment may aggregate or precipitate in some cases without being dissolved in the ink solvent, so that it has been extremely difficult to retain stable physical properties of an ink-jet ink. There is a method that electrothermal converters are used as ejection-energy supply means in particular to apply thermal energy to an ink so as to generate bubbles, thereby ejecting droplets of the ink. In this method, it is predicted that the physical properties of the ink greatly change within an ejection orifice and in the vicinity thereof, and so pigment particles have a tendency to easily aggregate within the ejection orifice and in the vicinity thereof. As a result, the ink droplets may have been ejected in an undesired direction in some cases, or the ejection orifice may have been closed in an extreme case.
In a pigment ink, the dispersion stabilization of pigment particles is achieved by causing a dispersion resin to be adsorbed on the surfaces of the pigment particles. The pigment particles dispersed by the dispersion resin are treated with the mechanism of dispersion/aggregation as colloidal particles like metal oxide fine particles. The theory thereof is stated in books (for example, Colloid Science I, Basis and Dispersion/Adsorption (TOKYO KAGAKU DOZIN CO., LTD.)). According to this book, fine particles in a liquid retain a dispersion condition or cause aggregation to the contrary due to a balance between repulsive force including electrostatic repulsive force and steric hindrance repulsive force and attractive force including van der Waals force.
In order to achieve the dispersion stabilization of a pigment, it has been proposed to use a graft polymer as the dispersion resin (see Japanese Patent Application Laid-Open Nos. H06-100810 and H10-87768). In these proposals, a graft polymer in which one of a main chain and a side chain is composed of a segment having high hydrophilicity and the other is composed of a segment having high hydrophobicity is used. The segment having high hydrophobicity is caused to be adsorbed on the surface of a pigment, and the segment having high hydrophilicity is brought into contact with an ink liquid, thereby enhancing electrostatic repulsive force and steric hindrance repulsive force to achieve dispersion stabilization.
At this time, the point for grafting the dispersion resin is that the segment having high hydrophobicity, which has a function of adsorbing on the surface of the pigment, and the segment having high hydrophilicity for improving the dispersion stability are separated in terms of the structure of the dispersion resin, whereby the respective functions are maximally developed. As described above, the structure of the resin is designed to achieve functional separation. Therefore, the hydrophobic segment favorably has higher hydrophobicity for enhancing the adsorptivity of the dispersion resin on the surface of the pigment, and the hydrophilic segment favorably has higher hydrophilicity for enhancing the dispersion stability of the pigment particles. On the other hand, a conventional ink used in ink jet recording generally contains water as a main component, to which a water-soluble high-boiling solvent such as glycol is added for the purpose of preventing clogging.
When such an ink has been used to conduct recording on plain paper, however, sufficient fixing ability has not been achieved. In addition, image irregularity may have occurred, which appears to be attributed to the uneven distribution of a filler or sizing agent on the surface of the recording medium. In particular, when a color image has been intended to form, a plurality of color inks has been applied one after another before they have been fixed to paper. Therefore, color bleeding and uneven color mixing have occurred at portions of boundaries between images of different colors (this phenomenon will hereinafter be referred to as “bleeding” simply), resulting in a failure to obtain satisfactory images.
As a method for improving color developability and resistance to bleeding, is mentioned a method in which the aggregating ability of pigment particles is improved after a pigment ink is impacted on the surface of paper. For example, there is a method in which the content of a hydrophilic monomer in a dispersion resin is lowered, thereby lowering the dispersion stability of pigment particles. In addition, as a means for improving resistance to bleeding and fixing ability to improve color developability, a recording method in which a reaction liquid having a high surface tension and containing a polyvalent metal salt is applied and a pigment ink having a low surface tension is then applied to conduct recording is disclosed (see Japanese Patent Application Laid-Open No. 2004-160996).
In a general recording method using no reaction liquid, when the dispersion stability of pigment particles is intended to be improved by a conventional technique, the aggregating ability of the pigment particles on the surface of a recording medium is lowered, so that the pigment particles tend to penetrate in the interior of the recording medium with the penetration of an ink. At this time, a phenomenon that a coloring material is partially repelled due to the uneven distribution of a filler or sizing agent on the surface of the recording medium occurs. A part of fiber on the surface of the recording medium in a printed region is exposed due to this phenomenon, so that sufficient color developability is not achieved. From this situation, it is understood that the dispersion stability of the pigment particles and the color developability become in a trade-off. On the other hand, in the method using the reaction liquid, the electrostatic repulsive force, which stabilizes the dispersion of the pigment particles, is reduced (substantially extinguished) by contact with the polyvalent metal salt, thereby aggregating the pigment particles. The aggregate fixes in the vicinity of the surface of the recording medium to cover the surface without exposing the fiber forming the recording medium in the printed region, thereby reducing bleeding and improving the color developability.
However, the pigment ink in which the pigment particles are dispersed by the dispersion resin whose hydrophobic segment and hydrophilic segment are separated for improving the dispersion stability as described above is strong in the electrostatic repulsive force and steric repulsive force acting between the pigment particles. As a result, the dispersion stability of the pigment particles is also improved, so that when such a pigment ink is applied to the recording method using the reaction liquid and the pigment ink, increase in the amount of pigment aggregate fixed to the surface of a recording medium cannot be expected owing to the improvement in dispersion stability, and so difficulty may be encountered on the improvement of color developability in some cases.
In a pigment ink wherein repulsive force for dispersion stabilization is generated by, for example, an anionic compound as a hydrophilic segment, the repulsive force for dispersion stabilization has been reduced by contact with the surface of a recording medium or a reaction liquid, thereby improving color developability. However, a phenomenon that the color developability is lower with the speeding-up of printing by a high-density recording head has been observed. The reason for this is that when droplets of the pigment ink ejected from the high-density recording head come into contact with the surface of the recording medium or the reaction liquid, a vigorous flow attending on the penetration of a liquid into the interior of the recording medium occurs on the surface of the recording medium as the printing is conducted at high speed from the recording head. An aggregate of the pigment particle formed by the contact of the surface of the recording medium or the reaction liquid with the pigment ink is disintegrated by such a vigorous flow of the liquid and finely pulverized. As a result, the pigment particles are easy to penetrate into the interior of the recording medium. A portion where the fiber on the surface of the recording medium is not covered with the pigment particles (is exposed) is increased. It is therefore considered that the color developability is lowered as the printing using the high-density recording head is speed up.
As described above, in the proposals to date, the pigment inks have been designed based on the notion of how aggregation is less caused for improving ejection stability from a recording head and storage stability. On the other hand, the mechanism of improving the color developability is based on the notion of how aggregation of the pigment ink is caused. Therefore, a pigment ink easier to aggregate has been able to improve the color developability. In other words, it has been necessary to add opposite properties to a pigment ink at the same time, and improvement in ejection stability and color developability of a recorded article have had the relationship of trade-off. Further, the pigment ink requires to have stronger aggregation force for conducting printing at high speed by the high-density recording head, so that it is more difficult to achieve both the improvement in ejection stability and the improvement in color developability. From the above, no pigment ink satisfying these all properties has been present.